The invention relates to apparatus for mixing light from different color lamps, in particular to produce white light.
The standard light source for small to moderate size narrow beam lighting for accent lighting and general illumination is the incandescent/halogen bulb, such as a PAR (parabolic aluminized reflector) lamp. These sources are compact and versatile, but they are not very efficient. A given lamp operates at a given color temperature for a fixed power, and while they are dimmable, the color temperature shifts with applied power according to the blackbody law, which may or may not be the variation that the user desires.
An array of LEDs in each of a plurality of colors offers the possibility of creating a luminaire in which the color temperature may be controlled at any power level, thereby enabling a lamp which is dimmable and emits a uniformly white light at any power level.
U.S. application Ser. No. 09/338,997 filed Jun. 24, 1999 discloses apparatus having beam splitters which mix light from an array of different color LEDs. A square array of four LEDs is arranged so that each pair emits light toward an opposite side of a semi-reflective layer at 45.degree., and a pair of mixed light beams is transmitted from each side at 45.degree.. A second identical stage mixes the two pairs of mixed beams so that four identical mixed beams emerge in parallel as white light. The semi-reflective surface may be approximated by a checkerboard pattern of fully reflective and fully transmissive areas which are sufficiently small that emerging sub-beams cannot be resolved and appear to be fully mixed.